Light pens are commonly used as input devices for data processing and storage systems in place of conventional keyboards and mouse devices. A light pen is used in combination with a display screen incorporating a photosensitive digitizer or photosensitive sheet material. As such, information can be input by writing with the light pen on the screen and typically the screen displays the instantaneous position, and path of movement, of the pen. In this way, the screen displays the pattern, e.g. a written message, sketch or signature traced thereon.
The light source for known light pens disadvantageously comprises a relatively fragile lamp or L.E.D. which can readily suffer damage if the pen is dropped. The light source is powered either by way of a power supply cord or cable or by way of a battery if a "cordless" light pen is required. The power is supplied by way of electrical connections which may also be fragile and prone to breakage if the pen is dropped or otherwise misused. Such components are generally expensive to purchase and assemble.
Cordless light pens are advantageous in that the movement and use of the pen is not impaired by a cord which can easily become snagged or otherwise interfere with the light pen. Also, a single cordless light pen can be used with a plurality of digitizer units, or other devices, since the pen is not anchored to one unit by a power supply cord.
However, known cordless light pens suffer particular disadvantages in that a battery must be provided for its use, which increases the operating cost of such pens. If a replaceable battery is used, this further increases the cost and also makes operation of the pen dependent on the availability of a replacement battery. If a re-chargeable battery is employed this may also lead to an increase in the "down time" for the pen, i.e. the period during which the pen can not be used.